1032 Sizing Of Components

The sizing of the components of the electrical system and the mechanical system starts once the drivetrain architecture is laid out based on the mission of the vehicle. In a series HEV, the electrical system design is the same as that of an EV. The IC engine size is specified for keeping the batteries charged. The sizing of the components of a parallel HEV is much more complex. If the vehicle is designed with heavier biasing on the IC engine, then the batteries can be downsized and reconfigured for maximum specific power instead of maximum specific energy. The battery and the motor serve to supply peak power demands during acceleration and over-taking without being discharged. The battery also acts as a reservoir for the regenerative braking energy. Ultracapacitors can be used instead of batteries, provided they meet the requirements during peak power demands. If the vehicle is to be more battery biased, then the system is configured such that the batteries will reach about 80% DoD at the end ofthe longest trip. Once the power requirements ofthe electrical and mechanical systems are apportioned for the parallel HEV, the electrical components are designed based on the power designated for the electrical system using the same design philosophy as that used for the EV components. A philosophy parallel to that used for EV can be used to design mechanical subsystems, where the components are sized based on initial acceleration, rated cruising velocity, maximum velocity, and maximum gradability .2 The gear ratio between the IC engine and the wheel shaft of a parallel HEV can be obtained by matching the maximum speed of the IC engine to the maximum speed of the driveshaft. A single gear transmission is desired to minimize complexity. The sizing of the components of a parallel HEV is discussed below.

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